This invention relates to engine exhaust systems, and particularly to a system for rapidly heating a catalytic converter or other exhaust processor to its minimum operating temperature at the beginning of a cold start cycle of an engine. More particularly, this invention relates to a system for reducing emissions from an exhaust system by conducting combustion product from the engine to heat a catalytic converter quickly to reduce the amount of time it takes for the catalytic converter to be heated from a cold start temperature to its minimum operating temperature during the initial moments of an engine cold start cycle.
Exhaust processors such as catalytic converters, particulate traps, and other contaminant filters are mounted on board a vehicle to clean and filter combustion product produced by the engine before the combustion product is discharged into the atmosphere through a tail pipe. A catalytic converter typically includes a catalyzed ceramic or metal substrate that is configured to purify the hot combustion product produced by the engine to remove certain contaminants from the engine exhaust. The catalytic converter treats the combustion product to produce an exhaust stream meeting stringent state and federal environmental regulations and emission standards.
Typically, hot combustion product is conducted through a pipe mounted under the body of a vehicle between an engine and a remote exhaust processor. The temperature of the combustion product decreases somewhat during this journey. At the beginning of a cold start of an engine, the exhaust processor is "cold" and typically has a temperature that is about equal to the temperature of the surroundings. Over time, the hot combustion product produced by a cold-started engine heats the substrate and housing in the exhaust processor to an operating temperature.
A catalyzed substrate purifies contaminants from engine combustion product most efficiently at high temperatures. However, a catalyzed substrate does not actively and efficiently treat combustion product until it is heated to a minimum operating temperature during the initial moments of an engine cold start cycle. A catalytic converter is said to "light off" when it is heated to its minimum operating temperature and begins to purify combustion product in an effective manner.
A substantial reduction in tail pipe emissions measured using the Federal Test Procedure can be realized by minimizing the elapsed time between engine ignition and catalytic converter light off during an engine cold start cycle. The majority of total emissions occurs during the cold start portion of the Federal Test Procedure before the catalytic converter has been heated to reach its minimum operating temperature. Accordingly, vehicle emissions can be reduced by achieving faster light off of the catalytic converter at the beginning of an engine cold start cycle.
With respect to the above-noted problem, U.S. Pat. No. 4,731,993 to Ito et al discloses a rear exhaust manifold having thick walls and a front exhaust manifold made of a thin stainless steel plate so that the front exhaust manifold has walls thinner than the walls of the rear exhaust manifold. It is also known from U.S. Pat. No. 5,018,66 to Cyb to apply a thin layer of heat-resistant compound to the interior of an exhaust manifold and from U.S. Pat. No. 5,004,018 to Bainbridge to provide an insulated exhaust pipe including inner and outer spaced tubes separated by refractory fiber insulation. Systems using electrically heated catalytic converters and catalytic converters containing increased amounts of precious metals are also known.
There is a need to improve vehicle emission controls to meet increasingly stringent emission standards. An exhaust system configured to provide quicker light off of the catalytic converter using heat energy contained in the hot combustion product produced by an engine would be an improvement over conventional exhaust systems.
Conventional exhaust systems typically use relatively thick-walled pipes made of heavy gauge metal to conduct combustion product from the exhaust manifold to the catalytic converter. The wall of a conventional cast iron manifold or pipe is typically 6.0 mm (0.236 inch) and the wall of a conventional stainless steel exhaust system pipe is 1.4-1.8 mm (0.055-0.071 inch). Because of the heavy gauge metal structure, these conventional pipes have a high "thermal capacitance." That is, the product of the mass and specific heat of these conventional combustion product-carrying pipes is quite large and they act as large heat sinks for hot combustion product in the exhaust system during the initial moments of an engine cold start cycle. For example, the thermal capacitance per unit length per unit diameter of a conventional pipe made of exhaust grade stainless steel and having a wall thickness of 1.78 mm (0.70 inch) is about ##EQU1##
As a result of the high thermal capacitance of the conventional exhaust system pipes, a large portion of the heat energy from the combustion product is consumed in heating the heavy gauge pipes. By allowing heat energy from the combustion product to be diverted to the pipes, less heat energy is available to heat the catalyzed substrate to its minimum operating temperature at the beginning of a cold start cycle of an engine.
It would be desirable to reduce the amount of heat energy used to heat pipes in an exhaust system during the initial moments of an engine cold start cycle. This conservation of heat energy would help to raise the temperature of the substrate to reach its minimum operating temperature in less time. Tail pipe emissions would be reduced if the substrate in an improved exhaust system reached its minimum operating temperature at an earlier point during an engine cold start cycle.
According to the present invention, an apparatus is provided for delivering combustion product from an engine to an exhaust processor which has an inlet and a predetermined minimum operating temperature. The apparatus includes a thin-walled pipe conducting hot combustion product from the engine to the inlet of the exhaust processor. The thin-walled pipe has a thermal capacitance per unit length per unit diameter of less than ##EQU2## to minimize the elapsed time during a cold start cycle of the engine until the inlet of the exhaust processor is heated by the combustion product to reach its predetermined minimum operating temperature. Advantageously, this thin-walled pipe has a low thermal capacitance and is responsible for reducing the time it takes to achieve light off by minimizing diversion of heat energy from the hot combustion product to the pipe as the combustion product is conducted from the engine to the exhaust processor during an engine cold start cycle so that total emissions are reduced.
Preferably, the thin-walled pipe includes a tubular wall having a wall thickness of less than 1.10 mm (0.043 inch) and is made of exhaust grade stainless steel which has a density of ##EQU3## and a specific heat of ##EQU4## Thus, the thin-walled pipe has a low thermal capacitance per unit length per unit diameter of less than ##EQU5## so it does not act as a significant heat sink to divert heat energy in the combustion product away from the inlet of the exhaust processor at the beginning of an engine cold start cycle. The thermal capacitance of a material is the product of the volume, density, and specific heat of the material. In a presently preferred embodiment, the wall thickness of the tubular side wall is 0.46 mm (0.018 inch) and has a low thermal capacitance per unit length per unit diameter of ##EQU6##
In preferred embodiments, the apparatus further includes an outer shell extending between the engine and the inlet of the exhaust processor. This outer shell is a large diameter pipe that surrounds the thin-walled pipe and is made of a relatively heavy gauge metal. The apparatus further includes means for supporting the thin-walled pipe inside the passageway formed in the outer shell in spaced-apart relation to the outer shell to establish an annular air gap therebetween. Advantageously, the outer shell and the supporting means are configured to protect and support the thin-walled pipe along its entire length between the engine and the exhaust processor without absorbing a lot of heat from engine combustion product at engine start-up.
In other preferred embodiments, the thin-walled pipe is positioned to extend into and through the exhaust manifold that is attached to the engine. The thin-walled pipe is supported in the interior region formed in the exhaust manifold to lie in spaced-apart relation to an interior wall of the exhaust manifold. Preferably, an annular air gap is formed around the thin-walled pipe to insulate the thin-walled pipe and reduce conductive heat transfer from the thin-walled pipe to the surrounding exhaust manifold.
The use of a thin-walled pipe having a low thermal capacitance to conduct hot combustion product from an engine to the inlet of an exhaust processor causes the heat energy in the combustion product to reach the exhaust processor in a shorter period of time during the start-up of a cold engine. This low thermal capacitance thin-walled pipe provides an improvement over pipes in conventional exhaust systems in that it causes the catalytic converter in the exhaust processor to be heated to its minimum operating temperature and light off more rapidly at the beginning of a cold start cycle of the engine. Consequently, the catalytic converter is active to lower total vehicle emissions without resorting to complex exhaust control mechanisms, costly exhaust system materials, or electrically preheated catalytic converters. Essentially, the low thermal capacitance thin-walled pipe conserves the heat energy already available in the hot combustion product discharged by the engine and uses that heat energy to effectively light off the catalytic converter very early in the cold start cycle of an engine and reduce total emissions and resulting pollution.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.